1. Field of the Invention
The present invention relates to an information reproduction technique using a ferromagnetic body and, more particularly, to a magnetic memory device using a magnetoresistive element and a method of manufacturing the same.
2. Description of the Related Art
A magnetic random access memory (to be abbreviated as an MRAM hereinafter) is a general term for solid-state memories which can rewrite, hold, and read out recording information at any time by using the magnetization direction of a ferromagnetic body as an information recording medium.
A memory cell of an MRAM normally has a structure formed by stacking a plurality of ferromagnetic layers. To record information, the relationship between the magnetization directions of the plurality of ferromagnetic bodies that form the memory cell is set to “parallel” or “anti-parallel”. Binary information “0” or “1” is recorded in correspondence with the parallel or anti-parallel state.
A write of recording information is executed by supplying a current to write lines which are laid out in a cross stripe and inverting the magnetization direction of the ferromagnetic body of each cell by a current magnetic field generated by the current. Since the MRAM is a nonvolatile memory, power consumption is zero during holding of recorded information in principle, and recorded information is held even after power-off.
On the other hand, a read of recorded information is executed by using a so-called magnetoresistive effect. In this phenomenon, the electrical resistance of a memory cell changes depending on the relative angle between a sense current and the magnetization direction of the ferromagnetic body of the cell or the relative angle in magnetization between the plurality of ferromagnetic layers.
The MRAM has a number of functional advantages as compared to a conventional semiconductor memory using a dielectric material. More specifically, (1) the MRAM is completely nonvolatile and can be rewritten 1015 times or more. (2) Since a nondestructive read can be executed, and no refresh operation is necessary, the read cycle can be shortened. (3) The resistance against radiation is high as compared to a charge accumulation type memory cell. The degree of integration per unit area and the write and read times of the MRAM would be almost the same as those of a DRAM. MRAMs which have nonvolatility as a remarkable characteristic are therefore expected to be applied to external recording devices for portable equipment, LSI embedded memories, and main memories of personal computers.
MRAMs which are presently considered for practical application use an element that exhibits a tunnel magnetoresistance effect (to be abbreviated as a TMR effect hereinafter) as a memory cell (e.g., Roy Scheuerlein, et al., “A 10ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, 2000ISSCC Digest of Technical Papers, U.S.A., February 2000, pp. 128-129). The element (to be referred to as an MTJ (Magnetic Tunnel Junction) element hereinafter) that exhibits the TMR effect is mainly constructed by a three-layered structure of ferromagnetic layer/ insulating layer/ferromagnetic layer. A current tunnels through the insulating layer. The tunnel resistance value changes in proportion to the cosine of the relative angle of the magnetization directions of the two ferromagnetic metal layers, and takes a maximal value when the magnetization directions are anti-parallel. For example, in a tunnel junction made of NiFe/Co/Al2O3/Co/NiFe, a magnetoresistance change rate more than 25% is observed in a low magnetic field of 50 Oe or less (e.g., M Sato, et al., “Spin-Value-Like Properties and Annealing Effect in Ferromagnetic Tunnel Junctions”, IEEE Trans. Mag., U.S.A., 1997, Vol. 33, No. 5, pp. 3553-3555). As a structure of an MTJ element, a so-called spin valve structure is known in which an anti ferromagnetic body is arranged adjacent to one ferromagnetic body to fix the magnetization direction to increase the magnetic field sensitivity (e.g., M Sato, et al., “Spin-Value-Like Properties and Annealing Effect in Ferromagnetic Tunnel Junctions”, Jpn. J. Appl. Phys., 1997, Vol. 36, Part 2, pp. 200-201). Another known structure has two tunnel barriers to improve the bias dependence of the magnetoresistance change rate (e.g., K Inomata, et al., “Spin-dependent tunneling between a soft ferromagnetic layer and hard magnetic nano particles, Jpn. J. Appl. Phys., 1997, Vol. 36, Part 2, pp. 1380-1383).
When the above MTJ element is applied to an MRAM, each memory cell has a planar structure shown in FIG. 57 or 58. As shown in FIG. 57 or 58, MTJ elements 19 are laid out at the intersections of and between word lines 10 and bit lines 23. A switching element (not shown) such as a MOS transistor is connected to the lower surface of each MTJ element 19 through a lower metal layer 13 and contact 12.
In the prior-art MRAM, the lower metal layer 13 is formed such that it exists even outside the side surfaces of the MTJ element 19 and contact 12. That is, a margin is generated in consideration of misalignment to the MTJ element 19 and contact 12. To separate adjacent cells, the lower metal layers 13 are formed at minimum pitches A and B. Under these circumstances, it is difficult to reduce a pitch X′ or X″ between the word lines 10 or a pitch Y′ or Y″ between the bit lines 23 by a predetermined amount or more. This problem becomes more conspicuous as the cell size is required to be smaller.